The present invention relates to a radio communication apparatus, and more particularly, to a radio communication apparatus in a digital mobile communication.
As a transmission system in a mobile communication, Frequency Division Duplex (hereinafter referred to as xe2x80x9cFDDxe2x80x9d) system and Time Division Duplex (hereinafter referred to as xe2x80x9cTDDxe2x80x9d) system are two leading examples.
The FDD system is a communication system that uses different frequency bands (here, f0 and f1) for the up link (hereinafter referred to as xe2x80x9cULxe2x80x9d) and down link (hereinafter referred to as xe2x80x9cDLxe2x80x9d) as shown in FIG. 10A, while the TDD system is a communication system that uses a same frequency band (here f2) for UL and DL as shown in FIG. 10B.
As a mobile communication, the TDD system, which uses fewer frequencies per channel, is currently considered to be a more promising system.
By the way, mobile communications are becoming increasingly faster with each passing day and a unit time (hereinafter referred to as xe2x80x9cslotxe2x80x9d) used for UL and DL in the TDD system tends to be shortened. For example, in the case where a transmission rate of 200 kbps is sped up to 20 Mbps, the time required to transmit a same amount of information is reduced to {fraction (1/100)}.
Since UL and DL use a same frequency in the TDD system, UL and DL have a same propagation environment that influences on them and in the case where the distance between UL and DL, that is, a one-slot length is very short, it is possible to assume that the propagation environment characteristic will not change between UL and DL. In this case, during transmission from a base station apparatus to a mobile station apparatus over DL, the base station apparatus can multiply an inverse characteristic of the propagation environment estimated during reception from the mobile station apparatus beforehand instead of ordinary equalization processing, thereby compensating influences from the propagation environment.
Using this method called xe2x80x9cpre-codingxe2x80x9d, it is not necessary for the mobile station apparatus to compensate influences from the propagation environment, making it possible to simplify and reduce the size of the mobile station apparatus.
The pre-coding will be explained in detail with reference to FIG. 11 below. FIG. 11 is a schematic diagram of a base station apparatus and mobile station apparatus that carry out a radio communication to explain the pre-coding.
In FIG. 11, suppose the propagation environment characteristic can be expressed by H (z). That is, signal x(z), which is transmitted from base station apparatus 1101 through this propagation environment, is multiplied by H(z) resulting in x(z)xc2x7H(z) when the transmission signal arrives at mobile station apparatus 1102. Therefore, in a system that uses ordinary equalization processing, it is necessary to remove the H(z) component from the reception signal as shown in FIG. 11A.
Thus, base station apparatus 1103 performs pre-coding as shown in FIG. 11B. That is, when base station apparatus 1103 receives a signal from mobile station apparatus 1104, if base station apparatus 1103 estimates that the propagation environment characteristic is H(z), base station apparatus 1103 multiplies transmission signal x(z) by an inverse characteristic of the propagation environment Hxe2x88x921(z) beforehand and sends x(z)xc2x7Hxe2x88x921(z) to mobile station apparatus 1104 right after the reception. Then, the signal that mobile station apparatus 1104 receives is x(z)xc2x7H31 1(z)xc2x7H(z)=x(z), which eliminates the need for mobile station apparatus 1104 to compensate to remove influences of the propagation environment.
Thus, in the TDD system in which transmission slots and reception slots are provided alternately in a same frequency band, the base station apparatus uses a propagation environment estimated by using unique words (hereinafter referred to as xe2x80x9cUWxe2x80x9d) included in the reception signal for pre-coding during next data transmission and thereby implements a radio communication without the need for the mobile station apparatus to compensate influences from the propagation environment and without being affected by the propagation environment.
However, the conventional base station apparatus has a problem of being unable to keep up with an ever-changing propagation environment.
That is, in the case of an asymmetric communication whose DL duration is longer than UL duration or in the case of a symmetric communication with a large interval of switching between UL and DL, once an empty slot is produced, the interval between the time of reception at which the propagation environment is estimated and the time of the next transmission at which the propagation environment information is applied to pre-coding will be extended, resulting in a problem that it is not possible to perform pre-coding according to a variation produced in the propagation environment in the meantime.
It is an object of the present invention to provide a radio communication apparatus carrying out pre-coding capable of keeping up with an ever-changing propagation environment in the case of an asymmetric communication whose DL duration is longer than UL duration or in the case of a symmetric communication with a large interval of switching between UL and DL.
A theme of the present invention is that in the case where an empty slot is produced with user data to be transmitted not assigned, the radio communication apparatus makes it possible to estimate the propagation environment by transmitting UW, a control channel signal, to the other end of communication using the empty transmission slot and by the other end of communication receiving this UW, thus carrying out accurate pre-coding in the case of an asymmetric communication whose DL duration is longer than UL duration or in the case of a symmetric communication with a large interval of switching between UL and DL.